Lower alkyl alcohol recovery from a stripping mixture

ABSTRACT

Methods of removing a lower alkyl alcohol from a polyester mixture of polyol fatty acid polyester and lower alkyl alcohol comprise (a) contacting the polyester mixture with a stripping mixture comprising an inert stripping gas, up to about 10,000 ppm lower alkyl alcohol and up to about 2000 ppm oxygen, wherein at least a portion of the lower alkyl alcohol is transferred from the polyester mixture to the stripping mixture, thereby increasing the concentration of lower alkyl alcohol in the stripping mixture, and (b) separating the stripping mixture from the polyester mixture. The methods further comprise (c) compressing the stripping mixture to increase its pressure, (d) cooling the stripping mixture to reduce its temperature thereby condensing at least a first portion of the lower alkyl alcohol to a liquid, (e) separating condensed lower alkyl alcohol from the stripping mixture to reduce the amount of lower alkyl alcohol in the stripping mixture to a range of from about 1 ppm to about 10,000 ppm, and (f) directing the resulting stripping mixture to an expansion turbine in which the temperature and pressure of the stripping mixture are reduced. Energy resulting from the decrease in the stripping mixture temperature and pressure in the expansion turbine is used to compress the stripping mixture separated from the polyester mixture.

This application is a 371 of PCT/US98/27275 filed Dec. 22, 1998, whichclaims the benefit of provisional application No. 60/072,388 filed Jan.9, 1998.

TECHNICAL FIELD

The present invention is directed to methods for removing lower alkylalcohol from polyol fatty acid polyesters by contacting the polyol fattyacid polyester and lower alkyl alcohol mixture with a stripping mixturecomprising an inert stripping gas, lower alkyl alcohol and oxygen. Aftersuch contact, the concentration of lower alkyl alcohol in the strippingmixture is increased. The invention is also directed to energy efficientmethods for removing the lower alkyl alcohol from the stripping mixtureby increasing the pressure and reducing the temperature of the strippingmixture so that a portion of the lower alkyl alcohol condenses to aliquid which can be more easily separated from the stripping mixture.

BACKGROUND OF THE INVENTION

There has been considerable interest in the use of certain polyol fattyacid polyesters as low or reduced calorie substitutes for fats and oilsin foods. For example, non-absorbable, non-digestible sugar fatty acidesters or sugar alcohol fatty acid esters having at least four fattyacid ester groups, with each fatty acid having from 8 to 22 carbonatoms, have been used as partial or full fat substitutes in low caloriefood compositions.

A number of different processes have been disclosed in the art forpreparing these highly esterified polyol fatty acid polyesters, inparticular sucrose polyesters. In general, a polyol, for examplesucrose, is reacted with a fatty acid lower alkyl ester in the presenceof a basic initiator catalyst to form a polyol fatty acid polyester.Emulsifiers, phase transfer catalysts and the like can be used topromote the reaction between the polyol and the fatty acid lower alkylester. Lower alkyl alcohol is a by-product of this reaction and itspresence in the reaction mixture tends to slow the reaction'sprogression. Additionally, the lower alkyl alcohol is generally not adesired component in the polyol fatty acid polyester product. Thus, itis desirable to remove the lower alkyl alcohol from the polyol fattyacid polyester to both produce a purified polyol fatty acid polyesterand to speed the reaction between a polyol and a fatty acid lower alkylester.

Moreover, oxygen is generally considered a poison in atransesterification reaction or in a mixture of a polyol fatty acidpolyester and lower alkyl alcohol due to its tendency to oxidizereactants and products and to degrade the reaction catalyst. Hence, itis desirable to minimize the amount of oxygen which is added to amixture of polyol fatty acid polyester and lower alkyl alcohol.

Lower alkyl alcohol can be removed from a polyol fatty acid polyester bysparging with an inert gas. This process is discussed in U.S. Pat. No.4,518,772 to Volpenhein, U.S. Pat. No. 3,963,699 to Rizzi et al. andU.S. Pat. No. 4,517,360 to Volpenhein. These patents disclose a methodof vacuum separation for the removal of the lower alkyl alcohol whereininert gas sparging is used as a supplement to the vacuum removal oflower alkyl alcohol.

Processes for removing volatile organics from inert gas streams aregenerally known. For example, U.S. Pat. No. 4,295,282 to Fox disclosesan open cycle heat pump system for recovering condensable solventsand/or heat from gas streams. This process is disclosed in conjunctionwith the removal of paint fumes and other volatile vapors from the gasstream. U.S. Pat. No. 4,480,393 to Flink et al. and U.S. Pat. No.5,152,812 to Kovach also disclose processes for recovering condensableorganic components from an inert gas stream.

Owing to the increased use of polyol fatty acid polyesters in foodproducts and the like, there is a continuing need to improve theefficiency of and reduce the costs of manufacturing the polyol fattyacid polyesters. Specifically, there is a continuing need for fullyintegrated processes which are both efficient and economical and whichcan provide for removal of lower alkyl alcohol from a polyol fatty acidpolyester mixture comprising a polyol fatty acid polyester and a loweralkyl alcohol.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide improved methods forremoving lower alkyl alcohol from a polyol fatty acid polyester mixtureusing a stripping mixture comprising an inert stripping gas, lower alkylalcohol and oxygen and subsequently removing a portion of the loweralkyl alcohol from the stripping mixture, whereby the stripping mixturemay be recycled for further use if desired.

It is an additional object of the present invention to provide suchprocesses for removing a sufficient amount of the lower alkyl alcoholfrom the stripping mixture to allow the gas to be recycled for furthercontact with a polyol fatty acid polyester reaction mixture and/orpolyester mixture containing lower alkyl alcohol.

In one embodiment, the present invention is directed to a method forremoving lower alkyl alcohol from a polyester mixture which comprisespolyol fatty acid polyester and lower alkyl alcohol. The polyestermixture is contacted by a stripping mixture comprising an inertstripping gas, up to about 10,000 ppm lower alkyl alcohol and up toabout 2000 ppm oxygen. During the contact, at least a portion of thelower alkyl alcohol is transferred from the polyester mixture to thestripping mixture, thereby increasing the concentration of lower alkylalcohol in the stripping mixture. The stripping mixture is thenseparated from the polyester mixture and the stripping mixture iscompressed to increase its pressure. Additionally, the stripping mixtureis cooled to condense at least a first portion of the lower alkylalcohol to a liquid. Finally, condensed lower alkyl alcohol is separatedfrom the stripping mixture to reduce the amount of lower alkyl alcoholin the stripping mixture to a range of from about 1 ppm to about 10,000ppm, and the resulting stripping mixture is directed to an expansionturbine in which the temperature and pressure of the stripping mixtureare reduced. The energy resulting from the decrease in the strippingmixture temperature and pressure in the expansion turbine is used tocompress the stripping mixture separated from the polyester mixture. Theresulting stripping mixture may optionally be used as a coolant in oneor more heat exchangers to cool an additional quantity of strippingmixture separated from the reaction mixture.

In a preferred embodiment, the present invention comprises a method forremoving a lower alkyl alcohol from a polyester mixture in a reactionmixture resulting from the reaction of polyol and fatty acid lower alkylester. The reaction mixture comprises reaction products including polyolfatty acid polyester and lower alkyl alcohol. The reaction mixture iscontacted by a stripping mixture comprising an inert stripping gas, upto about 10,000 ppm lower alkyl alcohol and up to about 2,000 ppmoxygen. During the contact, at least a portion of the lower alkylalcohol is transferred from the reaction mixture to the strippingmixture, thereby increasing the concentration of lower alkyl alcohol inthe stripping mixture. The stripping mixture is then separated from thereaction mixture and the stripping mixture is compressed to increase itspressure. Additionally, the stripping mixture is cooled to condense atleast a first portion of the lower alkyl alcohol to a liquid. Finally, aportion of the condensed first portion of lower alkyl alcohol isseparated from the stripping mixture to reduce the amount of lower alkylalcohol in the stripping mixture to a range of from about 1 ppm to about10,000 ppm, and the resulting stripping mixture is directed to anexpansion turbine in which the temperature and pressure of the strippingmixture are reduced. The energy resulting from the decrease in thestripping mixture temperature and pressure in the expansion turbine isused to compress the stripping mixture separated from the polyestermixture.

In another embodiment, the present invention comprises a method forsynthesizing polyol fatty acid polyester. Specifically, a polyol isreacted with fatty acid lower alkyl ester to form a reaction mixturewhich comprises polyol fatty acid polyester and lower alkyl alcohol. Thereaction mixture is contacted with a stripping mixture according to themethods described above.

The methods described herein provide the advantage of an integratedprocess for removing lower alkyl alcohol from a polyol fatty acidpolyester mixture, condensing the alcohol to a liquid and separating thecondensed alcohol from the stripping mixture, with significant energyrecovery. Surprisingly, it has been found that to remove lower alkylalcohol from a polyester mixture, an inert stripping gas which comprisesup to about 10,000 ppm of lower alkyl alcohol can be used to obtainsubstantial removal of the lower alkyl alcohol from the polyestermixture. Moreover, in many polyester mixtures oxygen is considered acontaminant and it has been determined that the stripping mixtures ofthe present invention can comprise up to about 2,000 ppm oxygen withoutsignificantly degrading the reaction performance.

Additionally, the stripping mixture may subsequently be vented to theatmosphere or recycled for contact with further polyol fatty acidpolyester. By using the cooled stripping mixture from which a portion ofthe lower alkyl alcohol has been condensed and separated as a coolant inat least one heat exchanger to cool an additional quantity of strippingmixture, additional energy and operational savings may be realized.

BRIEF DESCRIPTION OF THE DRAWING

The specification will be better understood from the followingdescription taken in conjunction with the accompanying drawing in which:

FIG. 1 is a schematic process flow diagram of a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION

The present invention will now be described in detail with reference tospecific embodiments. In accordance with the present invention, loweralkyl alcohol is removed from a mixture of polyol fatty acid polyesterand the alcohol. Typically, a polyol is reacted with a fatty acid loweralkyl ester to produce a polyol fatty acid polyester bytransesterification of the polyol, with the formation of a lower alkylalcohol as a by-product. During the transesterification reaction ofpolyol to form polyol fatty acid polyester, the production of loweralkyl alcohol by-product shifts the reaction equilibrium and slows thereaction. To achieve a high degree of transesterification, i.e., toesterify as many hydroxyl groups of the polyol as possible, it istherefore advantageous to remove the lower alkyl alcohol from thereaction mixture so that the reaction of the polyol does not reachequilibrium prior to obtaining the desired level of transesterificationof the polyol. As used herein, “reaction mixture” is intended to includea mixture comprising polyol fatty acid polyester and lower alkylalcohol. The reaction mixtures described herein can comprise numerousunreacted reactants, catalyst and by-products of side reactions whichoccur during the transesterification reaction. Moreover, as used herein,“polyester mixture” is intended to include mixtures comprising polyolfatty acid polyester and lower alkyl alcohol from any source and it isunderstood that the reaction mixture defined above is a subset of apolyester mixture as defined herein.

Polyol fatty acid polyesters produced by methods other thantransesterification may also contain lower alkyl alcohol, and it isoften desirable to remove the lower alkyl alcohol from the polyol fattyacid polyester to obtain a purified product. Hence, while the removal oflower alkyl alcohol is described herein primarily in conjunction with areaction mixture, it is understood that the lower alkyl alcohol removalmethods disclosed herein are applicable to the removal of lower alkylalcohol from polyol fatty acid polyesters regardless of the source ofthe polyol fatty acid polyester.

As used herein, the term “polyol” is intended to include any aliphaticor aromatic compound containing at least two free hydroxyl groups.Suitable polyols can be selected from the following classes: saturatedand unsaturated straight and branch chain linear aliphatics; saturatedand unsaturated cyclic aliphatics, including heterocyclic aliphatics; ormononuclear or polynuclear aromatics, including heterocyclic aromatics.Carbohydrates and non-toxic glycols are preferred polyols.Monosaccharides suitable for use herein include, for example, mannose,galactose, arabinose, xylose, ribose, apiose, rhamnose, psicose,fructose, sorbose, tagatose, ribulose, xylulose, and erythrulose.Oligosaccharides suitable for use herein include, for example, maltose,kojibiose, nigerose, cellobiose, lactose, melibiose, gentiobiose,turanose, rutinose, trehalose, sucrose and raffinose. Polysaccharidessuitable for use herein include, for example, amylose, glycogen,cellulose, chitin, inulin, agarose, zylans, mannan and galactans.Although sugar alcohols are not carbohydrates in a strict sense, thenaturally occurring sugar alcohols are so closely related to thecarbohydrates that they are also preferred for use herein. Natural sugaralcohols which are suitable for use herein are sorbitol, mannitol, andgalactitol.

Particularly preferred classes of materials suitable for use hereininclude the monosaccharides, the disaccharides and sugar alcohols.Preferred unesterified polyols include glucose, fructose, glycerol,polyglycerols, sucrose, zylotol, and sugar ethers. A particularlypreferred polyol is sucrose. Preferred unesterified polyols also includealkoxylated polyols such as alkoxylated glycerol, alkoxylatedpolyglycerols, sorbitol alkoxylated glycerines, alkoxylatedpolysaccharides, and linked alkoxylated polyols such as linkedalkoxylated glycerins. Polyols may be alkoxylated with C₃-C₆ epoxides,such as propylene oxide, butylene oxide, isobutylene oxide, and penteneoxide, to produce epoxide-extended polyols having an epoxylation indexminimum of at least about 2, preferably in the range of from about 2 toabout 8, as described in U.S. Pat. No. 4,816,613, incorporated herein byreference. Polyols may be also alkoxylated with an epoxide, preferably aC₃-C₁₀ 1,2-alkylene oxide, in the presence of a ring-openingpolymerization catalyst, as described in U.S. Pat. Nos. 5,399,729 and5,512,313, incorporated herein by reference.

Suitable alkoxylated polyols for use herein are described in U.S. Pat.Nos. 4,983,329; 5,175,323; 5,288,884; 5,298,637; 5,362,894; 5,387,429;5,446,843; 5,589,217; 5,597,605; 5,603,978 and 5,641,534, allincorporated herein by reference. Suitable alkoxylated polyols includealkoxylated sugar alcohols, alkoxylated monosaccharides, alkoxylateddisaccharides, alkoxylated polysaccharides, alkoxylated C₂-C₁₀ aliphaticdiols, and alkoxylated C₃-C₁₂ aliphatic triols. Preferred alkoxylatedC₃-C₁₂ aliphatic triols are alkoxylated glycerols, more preferred arepropoxylated glycerols, and particularly preferred are propoxylatedglycerols having from about 3 to about 21 moles of propylene oxide permole glycerol. Preferred alkoxylated polysaccharides are alkoxylatedpolysaccharides containing anhydromonosaccharide units, while morepreferred are propoxylated polysaccharides containinganhydromonosaccharide units, as described in U.S. Pat. No. 5,273,772,incorporated herein by reference. Preferred linked alkoxylated glycerinsinclude those comprising polyether glycol linking segments, as describedin U.S. Pat. No. 5,374,446, incorporated herein by reference, and thosecomprising polycarboxylate linking segments, as described in U.S. Pat.Nos. 5,427,815 and 5,516,544, incorporated herein by reference; morepreferred are those described in U.S. Pat. No. 5,516,544.

As used herein the term “polyol fatty acid polyester” is intended toinclude any polyol, as defined herein, which has two or more of itshydroxyl groups esterified with fatty acid groups. Suitable polyol fattyacid polyesters include sucrose polyesters having on average at leastfour, preferably at least about five, ester linkages per molecule ofsucrose; the fatty acid chains preferably have from about eight to abouttwenty-four carbon atoms. Other suitable polyol fatty acid polyestersare esterified linked alkoxylated glycerins, including those comprisingpolyether glycol linking segments, as described in U.S. Pat. No.5,374,446, incorporated herein by reference, and those comprisingpolycarboxylate linking segments, as described in U.S. Pat. Nos.5,427,815 and 5,516,544, incorporated herein by reference; morepreferred are those described in U.S. Pat. No. 5,516,544.

Additional suitable polyol fatty acid polyesters are esterifiedepoxide-extended polyols of the general formula P(OH)_(A+C) (EPO)_(N)(FE)_(B) wherein P(OH) is a polyol, A is from 2 to about 8 primaryhydroxyls, C is from about 0 to about 8 total secondary and tertiaryhydroxyls, A+C is from about 3 to about 8, EPO is a C₃-C₆ epoxide, N isa minimum epoxylation index average number, FE is a fatty acid acylmoiety and B is an average number in the range of greater than 2 and nogreater than A+C, as described in U.S. Pat. No. 4,861,613 and EP 0324010A1, incorporated herein by reference. The minimum epoxylation indexaverage number has a value generally equal to or greater than A and is anumber sufficient so that greater than about 95% of the primaryhydroxyls of the polyol are converted to secondary or tertiaryhydroxyls. Preferably the fatty acid acyl moiety has a C₇-C₂₃ alkylchain.

Preferred esterified epoxide-extended polyols for use herein includeesterified propoxylated glycerols prepared by reacting a propoxylatedglycerol having from 2 to about 100 oxypropylene units per glycerol withC₁₀-C₂₄ fatty acids or with C ₁₀-C₂₄ fatty acid esters, as described inU.S. Pat. Nos. 4,983,329 and 5,175,323, respectively, both incorporatedherein by reference. Also preferred are esterified propoxylatedglycerols prepared by reacting an epoxide and a triglyceride with analiphatic polyalcohol, as described in U.S. Pat. No. 5,304,665,incorporated herein by reference, or with an alkali metal or alkalineearth salt of an aliphatic alcohol, as described in U.S. Pat. No.5,399,728, incorporated herein by reference. More preferred are acylatedpropylene oxide-extended glycerols having a propoxylation index of aboveabout 2, preferably in the range of from about 2 to about 8, morepreferably about 5 or above, wherein the acyl groups are C₈-C₂₄,preferably C₁₄-C₁₈, compounds, as described in U.S. Pat. Nos. 5,603,978and 5,641,534, both incorporated herein by reference. Particularlypreferred are fatty acid-esterified propoxylated glycerols which exhibita sharp melt before about 92 F. (33 C.) and have a dilatomeric solid fatindex at 92 F. (33 C.) of less than about 30, as described in WO97/2260, or which have a dilatomeric solid fat index of at least about50 at 70 F. (21 C.) and at least about 10 at 98.6 F. (37 C.), asdescribed in U.S. Pat. Nos. 5,589,217 and 5,597,605, both incorporatedherein by reference.

Other suitable esterified epoxide-extended polyols include esterifiedalkoxylated polysaccharides. Preferred esterified alkoxylatedpolysaccharides are esterified alkoxylated polysaccharides containinganhydromonosaccharide units. more preferred are esterified propoxylatedpolysaccharides containing anhydromonosaccharide units, as described inU.S. Pat. No. 5,273,772, incorporated herein by reference.

In the transesterification of polyol in accordance with the presentinvention, the desired product preferably is a polyol fatty acidpolyester wherein at least half of the hydroxyl groups of the polyol arereplaced with fatty acid esters. When the polyol is sucrose, four ormore of the hydroxyl groups are desirably esterified. Even morepreferably, the desired product is a polyol fatty acid polyester whereinall of the hydroxyl groups are transesterified. As each hydroxyl groupof a polyol is transesterified, a molecule of lower alkyl alcohol isnormally produced. Thus, the reaction mixture comprises, among othercomponents, the desired polyol fatty acid polyester and the lower alkylalcohol.

To remove the lower alkyl alcohol from a reaction mixture as describedherein, a stripping mixture comprising an inert stripping gas, up toabout 10,000 ppm, more preferably up to about 3,000 ppm, lower alkylalcohol and up to about 2000 ppm is utilized oxygen. Surprisingly, ithas been found that a stripping mixture as defined herein whichcomprises up to about 10,000 ppm of a lower alkyl alcohol, can be use toremove lower alkyl alcohol from a polyester mixture. Moreover, thestripping mixtures defined herein can comprise up to about 2000 ppm ofoxygen which can be a contaminant in a polyester mixture. A preferredinert stripping gas for use in the present invention is nitrogen gas.However, other gases which are inert with respect to the polyol fattyacid polyester mixture, and in which the lower alkyl alcohol by-productis soluble can be utilized. Other inert stripping gases acceptable foruse with the present invention include hexane and other aliphatichydrocarbons.

To effect removal of the lower alkyl alcohol from a polyester mixturethe stripping mixture is contacted with the polyester mixture. Thecontact can take place, for example, in a reaction vessel or any otherappropriate vessel wherein the stripping mixture can contact thepolyester mixture. The stripping mixture can be fed co-current to thepolyol fatty acid polyester and lower alkyl alcohol mixture, orreferably the stripping mixture can be fed countercurrent to thepolyester mixture.

For a reaction mixture, it is preferable to contact the reaction mixturein the vessel where the reaction occurs. One preferred method forproducing the polyol fatty acid polyesters of the present invention isto provide a vertical multi-stage column for reacting the polyol and thefatty acid lower alkyl ester wherein the reaction components are fedinto the top of the column and are allowed to flow through the column tothe bottom where the product stream is removed. The stripping mixture isfed into the bottom of the column, bubbling through the column andexiting from the top of the column. One such contactor 4 isschematically shown in FIG. 1.

The present invention can be better understood by reference to FIG. 1which is a schematic representation of one method according to thepresent invention. Specifically, FIG. 1 illustrates a method wherein thepolyol fatty acid polyester and lower alkyl alcohol mixture 2 isprovided in a contactor 4 where the mixture 2 contacts a strippingmixture supplied via inlet 10. The mixture 2 of polyol fatty acidpolyester and lower alkyl alcohol can be directed to the contactor 4 viaan inlet stream 3. Alternatively, the contactor 4 can be a reactionvessel or the like in which the polyol fatty acid polyester-lower alkylalcohol mixture is formed, for example, from polyol and fatty acid loweralkyl ester supplied to the contactor 4 via one or more reactant inletstreams 3. Exiting the contactor 4 is a stream 5 of polyol fatty acidpolyester wherein a portion of lower alkyl alcohol has been removed bythe stripping mixture supplied via inlet 10. Also exiting the contactor4 is a stream 12 which comprises the stripping mixture with an increasedconcentration of lower alkyl alcohol. As shown in FIG. 1, the strippingmixture supplied via inlet 10 is fed counter-current to the polyol fattyacid polyester outlet stream 5, i.e., inlet 10 introduces the strippingmixture at or near the bottom of the contactor 4 and the strippingmixture is allowed to bubble through the polyester mixture 2 and exitthe top of the contactor 4 while the polyol fatty acid polyester-loweralkyl alcohol mixture having a reduced lower alkyl alcohol content ispreferably removed from the bottom of the contactor 4 via the stream 5.As can be appreciated, the amount of the stripping mixture introducedinto the contactor 4 can vary significantly, depending upon, forexample, the rate at which lower alkyl alcohol is produced or is presentin the polyol fatty acid polyester and the amount of lower alkyl alcoholwhich is to be removed.

A preferred range for the ratio of stripping mixture supplied via inlet10 to the polyol fatty acid polyester-lower alkyl alcohol mixture 2 isfrom about 0.1:1 to about 10:1, more preferably from about 0.5:1 toabout 10:1, by weight. A further preferred range is from about 1:1 toabout 5:1 stripping mixture to polyester mixture by weight. The inlettemperature of the stripping mixture is preferably close to that of thepolyester mixture 2. However, as can be appreciated, due to the lowthermal capacitance of gas, the temperature of the stripping mixturewill rapidly equilibrate to the temperature of the liquid streamcontaining the mixture of polyol fatty acid polyester and the loweralkyl alcohol 2. Moreover, the contactor 4 can be cooled and/or heated,as appropriate, to maintain the desired temperature and to insure thatthe temperature of stripping mixture reaches the temperature of thepolyester mixture 2.

The contactor 4 can be operated at a variety of pressures, butatmospheric or slightly elevated pressures, up to about 2500 mm Hg, arepreferred. As can be appreciated, subjecting contactor 4 to a vacuum orsubatmospheric pressures may promote the removal of the strippingmixture, which is in the form of a vapor or gas, via outlet 12, from theliquid polyester mixture 2. However, use of subatmospheric pressurespromotes the introduction of air which can be drawn into contactor 4through cracks, leaks or other openings in the contactor. Theintroduction of surrounding air to contactor 4 is undesirable becauseair contains an appreciable concentration of oxygen which is considereda poison to most polyester mixtures described herein. More specifically,oxygen is known to poison the catalyst used to promote thetransesterification of a polyol and a lower alkyl methyl ester to form apolyol fatty acid ester. Additionally, oxygen can promote side reactionswhich compete with both reactants and products in a transesterificationreaction which forms polyol fatty acid polyester. Both poisoning of thecatalyst and promoting side reactions can cause a degradation of thereaction performance. Moreover, oxygen can serve to degrade the desiredpolyol fatty acid polyester in a polyester mixture. Hence, it isespecially preferred that the stripping mixture, for example stream 10entering contactor 4, have a concentration of oxygen of up to about 2000parts per million. It is often impractical and uneconomical to reducethe concentration of oxygen in a stripping mixture below about 1 partper million. However, often it is not necessary to remove any oxygenfrom the stripping mixture after it exits the contactor because anappreciable amount of oxygen can be consumed during contact with thepolyester mixture as discussed above. Thus, an especially preferredrange of oxygen in stripping mixture 10 is between about 1 ppm and about2000 ppm.

In contactor 4, at least a portion of lower alkyl alcohol is transferredfrom the polyester mixture to the stripping mixture. Thus, theconcentration of lower alkyl alcohol in the stripping mixture exitingvia stream 12 has a higher concentration of lower alkyl alcohol than thestripping mixture entering contactor 4 via stream 10. After thestripping mixture exits the contactor 4, it is fed through processequipment which increase the pressure and reduces the temperature of thestripping mixture, thereby causing at least a portion and preferablyessentially all of the lower alkyl alcohol to condense into a liquidstate. Those process steps are described in detail below.

After exiting contactor 4, a demister (not shown) and/or chiller (notshown) can be used to remove a portion of the liquid droplets which maybe entrained in the stripping mixture in line 12 and to reduce thetemperature of the stripping mixture in line 12, respectively. Toincrease the pressure of the stripping mixture, any appropriatepressurizing means can be used, for example, compressors 50 and 56 shownschematically in FIG. 1. The pressure of the stripping mixture exitingcompressor 50 via stream 14 can be raised up to about 1700 mm Hg andpreferably up to about 2100 mm Hg. Compatibility with the strippingmixture is necessarily required for any compressor used and eachcompressor should be properly sized to handle the flow rates ofstripping mixtures in lines 12 and 18.

One or more heat exchangers, indicated as 52, can be used to reduce thetemperature of the stripping mixture in line 14 after it exits thecompressor 50. Any of a variety of commonly available heat exchangerscan be used, provided that they are compatible with the strippingmixture. Heat exchangers in general, and heat exchangers suitable foruse in the present invention, require a coolant, provided via stream 80.The temperature of the stripping mixture in line 14 is lowered as itflows in heat exchange relation with the coolant. The coolant istypically fed counter-current to the stripping mixture in line 14,although co-current flow is also acceptable. Cold water, glycols andother coolants known to the art are acceptable coolants for use with themethods described herein. Necessarily, the temperature of the coolant instream 80 is increased as it flows through and exits the heat exchangervia stream 82. One skilled in the art would recognize that the locationand number of heat exchangers and compressors are a design considerationwhich can be varied without diverging from the methods of the presentinvention.

The increase in pressure and decrease in temperature of strippingmixture 16 can cause at least a portion of the lower alkyl alcohol fromthe mixture to condense to a liquid. Alternatively, or in addition tocondensing a portion of the lower alkyl alcohol contained in stream 16,vaporized polyol fatty acid polyester contained in stream 16 can becondensed as the stripping mixture is passed through the heat exchanger52. The temperature and pressure necessary to condense the lower alkylalcohol will depend primarily on the type and concentration of loweralkyl alcohol being stripped from the polyol fatty acid polyester. Forexample, preferred fatty acid lower alkyl esters for use in theproduction of polyol fatty acid polyester of the present invention arefatty acid methyl esters. When fatty acid methyl esters react withpolyol to remove a hydroxyl group from the polyol, replacing it with afatty acid ester, methanol is produced. Thus, methanol would be strippedfrom the polyol fatty acid polyester mixture by the stripping mixture.At atmospheric pressure, the boiling point of methanol is approximately64 C. The temperature of the stripping mixture must be reduced to atleast below about 64 C. to condense some of the methanol at atmosphericpressure. However, because the stripping mixture is compressed by themethods described herein, i.e, the pressure of the gas mixture isincreased, the boiling point of the alcohol is subsequently raised.

As will be appreciated by those skilled in the art, condensation of avapor to its liquid form begins at the vapor's boiling point, but oftennot all of the vapor will condense at that temperature. Therefore, it ispreferred, and often necessary, to reduce the temperature of thestripping mixture to levels significantly below the boiling point of thelower alkyl alcohol to remove an appreciable quantity of alcohol fromthe stripping mixture. It is preferred to remove at least about 90% byweight, and more preferably 99% by weight, of the alcohol from thestripping mixture.

To ensure essentially complete removal of the lower alkyl alcohol fromthe stripping mixture, the temperature of the stripping mixture ispreferably reduced to below about −35 C. and more preferably to belowabout −65 C. so that essentially all of the lower alkyl alcohol iscondensed to a liquid. Condensing “essentially all” of the lower alkylalcohol means condensing greater than about 99% by weight of the loweralkyl alcohol present in the stripping mixture. Preferably, however, forthe stripping mixture to be suitable for further contact with apolyester mixture, the amount of lower alkyl alcohol should be reducedto below about 10,000 ppm, more preferably to below about 3000 ppm. Evenmore preferably, the lower alkyl alcohol in the striping mixture isreduced to below about 200 ppm and most preferably below about 20 ppmbefore the stripping mixture is recycled for further contact with thepolyester mixture. As can be appreciated, to reduce the amount of loweralkyl alcohol in the stripping mixture to below about 1 ppm may becostly and/or impractical. Hence, a preferred range of lower alkylalcohol in the stripping mixture prior to contact with a polyestermixture is between about 1 ppm and 10,000 ppm of lower alkyl alcohol.

Separation of the lower alkyl alcohol condensed liquid from thestripping mixture in stream 16 can be accomplished by any appropriatemeans, as shown schematically by separator 54, wherein the strippingmixture containing condensed lower alkyl alcohol flows into theseparator 54 via the stream 16. The collected liquid alcohol is removedfrom the separator 54, via stream 17, as the stripping mixture flowsthrough the separator. The stripping mixture leaves the separator 54 viaa stream 18 and is directed to a compressor 56 for additionalcompression up to about 3700 mm Hg, in the preferred embodiment of thepresent invention shown in FIG. 1. Examples of suitable separators foruse herein include, but are not limited to, fiber mist eliminator,impingement separator, gravity separator, and/or mesh pad separator.

After the stripping mixture is further compressed, it exits thecompressor 56 via a stream 20. In the preferred embodiment of thepresent invention shown in FIG. 1, the stripping mixture in stream 20 isdirected to two heat exchangers, 58 and 60, wherein the coolant streams,30 and 32 of the heat exchanger comprise stripping mixture which hasbeen cooled by expansion through an expansion turbine 64. Morespecifically, the stripping mixture in stream 20 exiting the compressor56 passes through the heat exchanger 58 and exits via a stream 22. Thestripping mixture in stream 22 is then directed to a heat exchanger 60where it exits via stream 24. The striping mixture in stream 24, afterhaving been chilled to a temperature preferably below about −35 C. tocondense a second portion of the lower alkyl alcohol, is fed to aseparator 62 wherein at least a portion of the second condensed portionof lower alkyl alcohol is removed from the stripping mixture as liquidalcohol stream 25. Suitable separators include, for example, fiber misteliminator, impingement separator, gravity separator, and/or mesh padseparator. The stripping mixture exits separator 62 via a stream 26 andis then passed through the expansion turbine 64. The striping mixtureexits the expansion turbine 64 via a stream 28.

The expansion turbine 64 generates energy from the expanding strippingmixture. Specifically, the pressurized stripping mixture drives theturbine 64, thus, reducing the pressure, and subsequently reducing thetemperature, of the stripping mixture wherein the reduction in pressureand temperature is transformed into mechanical energy. Preferably, thetemperature and pressure of the stripping mixture contained in stream 26are reduced to below about −50 C. and to below about 1500 mm Hg, andmore preferably to below about −75 C. and to below about 1800 mm Hg.Energy generated by the reduction in pressure and temperature isadvantageously used to compress the stripping mixture separated from thepolyol fatty acid polyester. A shaft 51 can be connected to the turbine64 so that the stripping mixture 26 entering the expansion turbine 64drives the shaft 51 wherein the rotating shaft can provide power forother mechanical apparatuses, for example, a portion of power to drivethe compressors 50 and 56 which are used to compress the strippingmixture in the streams 12 and 18, respectively. The increase in thepressure of the stripping mixture streams 12 and 18 in compressors 50and 56, driven by expansion turbine 64, will not be equal to thepressure drop of stripping mixture 26 passing through expansion turbine64 as some energy will necessarily be lost to friction and other energylosses. One skilled in the art will recognize that the configuration ofone expansion turbine connected to two compressors by a common shaft isa matter of design preference and other configurations, for example aplurality of expansion turbines and one compressor connected by a commonshaft, are possible and will be apparent to those skilled in the art.

While in the expansion turbine 64, both the temperature and pressure ofthe stripping mixture are reduced. By this method, the stripping mixturein stream 28 can then be utilized as the coolant in heat exchangers 60and 58. The stripping mixture in stream 28 is preferably passed througha demister 66 wherein fine droplets of entrained liquid are physicallyseparated from the stripping mixture which exits the demister 66 asliquid stream 29. A commercially available Brownian fiber demister ispreferred for use with the methods described herein, although otherdemisters may also be employed. The stripping mixture exits the demister66 via a stream 30. The stripping mixture in stream 30 serves as thecoolant for the heat exchanger 60 and therefore the temperature of thestripping mixture in stream 30 is increased as it exits the exchanger 60via a stream 32. The stripping mixture in stream 32 is utilized as thecoolant for the heat exchanger 58 wherein the temperature of thestripping mixture is increased. The stripping mixture exits heatexchanger 58 via a stream 34.

After flowing through the heat exchangers 60 and 58, the strippingmixture in stream 34 can be vented to the atmosphere via a vent (notshown). Venting a portion of the stripping mixture, for example 1.0-10%,and replacing it with an inert stripping gas which is relatively free ofoxygen, is often desirable to reduce the amount of oxygen in thestripping mixture prior to further contact with a polyester mixture.Alternatively, or in addition, the stripping mixture can be recycled tothe polyol fatty acid polyester contactor for further contact with thepolyester or reaction mixture and/or recycled to the stripping mixturein stream 12 in order to reduce the temperature of the stripping mixturein stream 12. Any combination of these three uses for the strippingmixture in stream 34 can be employed. From an economic point of view, itis desirable to minimize the amount of the stripping mixture 34 which isvented to the atmosphere so as to maintain steady state flow conditions.If the stripping mixture in stream 34 is intended for recycle forfurther contact with a polyester mixture, any stripping mixture which isvented to the atmosphere must be replaced by an essentially equal amountof stripping mixture to maintain steady state flow through the contactor4.

EXAMPLE I

The foregoing Detailed Description can be better understood by referenceto the following example wherein Table I sets forth preferred processparameters according to one embodiment of the present invention. Thestream numbers, 10 through 34, correspond with the streams shown in FIG.1. Specifically, the stripping mixture in stream 10 which is fed intothe contactor 4 to contact a polyester mixture 2 comprises primarilynitrogen and 18 ppm methanol. As can be seen, the stripping mixturewhich exits contactor 4 via stream 12 contains over 1200 lbs/hr ofmethanol in addition to the nitrogen. An inline cooler (not shown) anddemister (not shown) are utilized to reduce the temperature of strippingmixture in stream 12 to about 110 F. (45 C.) while removing a portion ofany liquid droplets which become entrained in the vapor phase of thestripping mixture. The stripping mixture in stream 12 is then compressedfrom a pressure of 0.9 psig to about 24.41 psig in the compressor 50which simultaneously increases the temperature of the stripping mixtureto about 320 F. (160 C.). The stripping mixture exits compressor 50 viathe stream 14.

The temperature of the stripping mixture in stream 14 is reduced in heatexchanger 52 to about 110 F. (45 C.) and the stripping mixture exits thecompressor 50 via the stream 16. The stripping mixture from stream 16 ispassed through separator 54 which at a temperature of 110 F. (45 C.)removes essentially no methanol from the stripping mixture. However,polyol fatty acid polyester and other organic materials having boilingpoints higher than methanol may be condensed and separated from thestripping mixture via the separator 54. The stripping mixture exitsseparator 54 via stream 18 and is further compressed in compressor 56 toa pressure of about 55.3 psig, which increases the temperature ofstripping mixture 18 to about 250 F. (120 C.). The temperature ofstripping mixture in stream 20 is then reduced in heat exchanger 58, andthe stripping mixture exits via stream 22 at a temperature of about 78F. (25 C.). Subsequently, the temperature of the stripping mixture instream 22 is reduced as it is passed through heat exchanger 60, and thestripping mixture exits via stream 24 at a temperature of about −49 F.(−45 C.).

Stream 24 is then passed through separator 62 wherein approximately 1200lb/hr of liquid methanol is removed from the stripping mixture whichexits separator 62 via liquid stream 27. This represents a removal rateof approximately 98% of the methanol in separator 62. As can be seen,the stripping mixture in stream 26 exiting separator 62 containsapproximately 300 ppm of methanol. As can be appreciated, the pressureof stream 26 is slightly lower than stream 20 due to frictional lossesincurred as the stripping mixture is passed through the two heatexchangers and the separator. The stripping mixture 26 is then expandedin expansion turbine 64 as described above to decrease both thetemperature and the pressure of the stripping mixture in stream 26 toabout −102 F. (−75 C.) and about 20.6 psig. The stripping mixture exitsexpansion turbine 64 via stream 28.

At this point in the process, an additional amount of methanolcondenses, but due to the extremely low concentration of methanol it isoften difficult to remove it in a standard separator. Hence, it ispreferred to use a fiber mist eliminator as the demister 66 whereinentrained droplets of liquid can be physically separated from thegaseous stream, i.e., the stripping mixture in stream 28, and thencollected and removed as a liquid, for example via stream 29. Thestripping mixture which exits demister 66 via stream 30 containsapproximately 18 parts per million of methanol which representsapproximately 0.1% of the original amount of methanol removed from thepolyester mixture. The stripping mixture in stream 30 is utilized as thecoolant in heat exchanger 60 to cool the stripping mixture in stream 22as described above. The temperature of the stripping mixture in stream30 is increased in heat exchanger 60 from about −102 F. (−75 C.) toabout 50 F. (10 C.), and exits via stream 32. The stripping mixture instream 32 is used as the coolant for heat exchanger 58 to cool thestripping mixture in stream 20 wherein the temperature of the strippingmixture in stream 32 is increased from about 50 F. (10 C.) to about 234F. (110 C.) and exits via stream 34. An optional heater 68 whichincreases the temperature of the stripping mixture in stream 34 fromabout 234 F. (110 C.) to about 275 F. (135 C.) is shown. Heater 68 canbe an electrical heater or an additional heat exchanger and is utilizedto prepare the stripping mixture in stream 10 for further contact with apolyester mixture in contactor 4.

TABLE I Stream 10 12 14 16 18 Methanol, 1.6 1267.6 1267.6 1267.6 1267.6lb/hr  (18 ppm) Nitrogen, 89697.1 89698.4 89698.4 89698.4 89698.4 lb/hrTemp., F. 275 110 320 110 110 Pressure, 15.30 0.90 24.41 24.00 22.41psig Density, 0.11 0.07 0.13 0.14 0.17 lb/ft³ Stream 20 22 24 26 25Methanol, 1267.6 1267.6 1267.6 26.2 1241.4 lb/hr (300 ppm) Nitrogen,89698.4 89698.4 89697.2 89697.2 1.3 lb/hr Temp., F. 250 70 49 −49 −49Pressure, 55.30 51.90 49.80 49.00 49.00 psig Density, 0.26 0.33 0.410.40 53.10 lb/ft³ Stream 28 30 29 32 34 Methanol 26.2 1.6 24.6 1.6 1.6lb/hr (300 ppm) (18 ppm)  (18 ppm) (18 ppm) Nitrogen 89697.2 89697.1 0.089697.1 89697.1 lb/hr (600 ppm) Temp. F. −102 −102 −102 50 234 Pressurepsig 20.60 20.10 20.10 18.00 15.90 Density 0.26 0.25 54.65 0.17 0.12lb/ft³

Having shown and described the preferred embodiments of the presentinvention, further adaptations of the methods described herein can beaccomplished by appropriate modifications by one of ordinary skill inthe art without departing from the scope of the present invention.Several of these potential modifications have been mentioned, and otherswill be apparent to those skilled in the art. For example, while twocompressors are shown processing the stripping mixture, one compressoror more than two compressors can be used. Moreover, as discussed above,the number and placement of heat exchangers can be varied and while itis envisioned that the stripping mixture will be reduced to a very lowtemperature, i.e., −35 C. or more, it is understood that the methods ofremoving lower alkyl alcohol from a stripping mixture can beaccomplished at higher temperatures, although less alcohol may beremoved. Accordingly, the scope of the present invention should beconsidered in terms of the following claims and understood not to belimited to the drawings of the processes and methods shown and describedin the specification and drawings.

What is claimed is:
 1. A method of removing a lower alkyl alcohol from apolyester mixture comprising polyol fatty acid polyester and lower alkylalcohol, the method comprising the steps of: (a) contacting thepolyester mixture with a stripping mixture comprising an inert strippinggas, up to about 10,000 ppm lower alkyl alcohol and up to about 2,000ppm oxygen, wherein at least a portion of the lower alkyl alcohol istransferred from the polyester mixture to the stripping mixture, therebyincreasing the concentration of lower alkyl alcohol in the strippingmixture, (b) separating the stripping mixture from the polyestermixture, (c) compressing the stripping mixture to increase its pressure,(d) cooling the stripping mixture to reduce its temperature, therebycondensing at least a first portion of the lower alkyl alcohol to aliquid, (e) separating at least a portion of the condensed loweralkyl-alcohol from the stripping mixture to reduce the amount of loweralkyl alcohol in the stripping mixture to a range of from about 1 ppm toabout 10,000 ppm, and (f) directing the resulting stripping mixture toan expansion turbine in which the temperature and pressure of thestripping mixture are reduced, wherein energy resulting from thedecrease in the stripping mixture temperature and pressure in theexpansion turbine is used to compress the stripping mixture separatedfrom the polyester mixture.
 2. The method according to claim 1, whereinafter step (e) but before step (f) a second portion of lower alkylalcohol is condensed in the stripping mixture and condensed lower alkylalcohol is separated from the stripping mixture.
 3. The method accordingto claim 1, wherein in step (d) the stripping mixture is passed throughat least two heat exchangers having a coolant flowing therethrough whichreduces the temperature of the stripping mixture to less than about −35°C.
 4. The method according to claim 3, wherein at least one of the heatexchangers comprises a water coolant which enters the heat exchanger ata temperature below about 100° F.
 5. The method according to claim 3,wherein the stripping mixture exiting the expansion turbine is used asthe coolant in at least one of the heat exchangers.
 6. The methodaccording to claim 1, wherein the stripping mixture exiting theexpansion turbine is recycled for contact with a polyester mixturecomprising polyol fatty acid polyester and lower alkyl alcohol.
 7. Themethod according to claim 2, wherein after the condensed second portionof lower alkyl alcohol is removed from the stripping mixture, thestripping mixture is recycled for contact with a polyester mixturecomprising polyol fatty acid polyester and lower alkyl alcohol.
 8. Themethod according to claim 1, wherein the lower alkyl alcohol ismethanol.
 9. The method according to claim 1, wherein the polyol fattyacid polyester is a sucrose fatty acid polyester.
 10. The methodaccording to claim 1, wherein the expansion turbine is mechanicallylinked to a plurality of compressors and the energy generated by thedecrease in the stripping mixture temperature and pressure in theexpansion turbine is transferred to the plurality of compressors toincrease the pressure of the stripping mixture after it is separatedfrom the polyester mixture.
 11. The method according to claim 2, whereinafter the condensed second portion of lower alkyl alcohol is removedfrom the stripping mixture, at least a portion of the stripping mixtureis vented to the atmosphere.
 12. The method according to claim 1,wherein the stripping mixture has a concentration of less than about 200ppm of lower alkyl alcohol after the condensed first portion of loweralkyl alcohol has been separated from the stripping mixture.
 13. Themethod according to claim 1, wherein greater than about 99% of the loweralkyl alcohol transferred from the polyester mixture to the strippingmixture is removed from the stripping mixture when the condensed firstportion of lower alkyl alcohol is separated from the stripping mixture.14. The method according to claim 1, wherein in step (a) the inert gasis nitrogen.
 15. The method according to claim 7, wherein before thestripping mixture is recycled for contact with the polyester mixture, atleast a portion of the stripping mixture is diverted and combined withthe separated stripping mixture from step (b).
 16. A method of removinga lower alkyl alcohol from a reaction mixture resulting from atransesterification reaction of a polyol and a fatty acid lower alkylester, the reaction mixture comprising a polyol fatty acid polyester andthe lower alkyl alcohol, the method comprising: contacting the reactionmixture with a stripping mixture comprising an inert stripping gas, upto about 10,000 ppm lower alkyl alcohol and up to about 2,000 ppmoxygen, wherein at least a portion of the lower alkyl alcohol istransferred from the reaction mixture to the stripping mixture, therebyincreasing the concentration of lower alkyl alcohol in the strippingmixture, separating the stripping mixture from the reaction mixture,compressing the stripping mixture to increase its pressure, cooling thestripping mixture to reduce its temperature, thereby condensing at leasta first portion of the lower alkyl alcohol to a liquid, separatingcondensed lower alkyl alcohol from the stripping mixture to reduce theamount of lower alkyl alcohol in the stripping mixture to a range offrom about 1 ppm to about 10,000 ppm, and directing the resultingstripping mixture to an expansion turbine in which the temperature andpressure of the stripping mixture are reduced, wherein energy resultingfrom the decrease in the stripping mixture temperature and pressure inthe expansion turbine is used to compress the stripping mixtureseparated from the reaction mixture.
 17. A method for synthesizing apolyol fatty acid polyester comprising the steps of: (a) reacting apolyol and a fatty acid lower alkyl ester to produce a reaction mixturecomprising polyol fatty acid polyester and lower alkyl alcohol, (b)contacting the reaction mixture with a stripping mixture comprising aninert stripping gas, up to about 10,000 ppm lower alkyl alcohol and upto about 2000 ppm oxygen, wherein at least a portion of the lower alkylalcohol is transferred from the reaction mixture to the strippingmixture, thereby increasing the concentration of lower alkyl alcohol inthe stripping mixture, (c) separating the stripping mixture from thereaction mixture, (d) compressing the stripping mixture to increase itspressure, (e) cooling the stripping mixture to reduce its temperaturethereby condensing at least a first portion of the lower alkyl alcoholto a liquid, (f) separating condensed lower alkyl alcohol from thestripping mixture to reduce the amount of lower alkyl alcohol in thestripping mixture to a range of from about 1 ppm to about 10,000 ppm,and (g) directing the resulting stripping mixture to an expansionturbine in which the temperature and pressure of the stripping mixtureare reduced, wherein energy resulting from the decrease in the strippingmixture temperature and pressure in the expansion turbine is used tocompress the stripping mixture separated from the reaction mixture. 18.The method according to claim 17, wherein in step (e) the strippingmixture is passed through at least two heat exchangers having a coolantflowing therethrough which reduces the temperature of the strippingmixture to less than about −35° C.
 19. The method according to claim 17,wherein the lower alkyl alcohol is methanol.
 20. The method according toclaim 18, wherein the stripping mixture exiting the expansion turbine isused as the coolant in at least one of the heat exchangers.
 21. Themethod according to claim 1, wherein after step (f) the strippingmixture is passed through a demister to separate from the strippingmixture at least a portion of any entrained droplets of liquid.
 22. Themethod according to claim 21, wherein the demister is a fiber misteliminator.
 23. The method according to claim 1, wherein the polyolfatty acid polyester is selected from the group consisting of esterifiedlinked alkoxylated glycerins, esterified epoxide-extended polyols andmixtures thereof.
 24. The method according to claim 1, wherein in step(d) the stripping mixture is passed through a heat exchanger having acoolant flowing therethrough which reduces the temperature of thestripping mixture to less than about −35° C.
 25. The method according toclaim 24, wherein said coolant is the stripping mixture exiting theexpansion turbine.
 26. The method according to claim 17, wherein in step(e) the stripping mixture is passed through a heat exchanger having acoolant flowing therethrough which reduces the temperature of thestripping mixture to less than about −35° C.
 27. The method according toclaim 26, wherein said coolant is the stripping mixture exiting theexpansion turbine.